1. Field of the Invention
The present invention relates to a paper feeding device that stores a paper stack and that feeds paper sheets, and to an image forming apparatus that comprises the paper feeding device.
2. Description of the Related Art
Paper feeding devices used in image forming apparatuses such as multifunction machines, fax machines, printers or the like, are configured in such a manner that paper is fed out, sheet by sheet, out of a paper stack that is stored in the paper feeding device. The fed-out paper sheet is fed to an image forming unit in the image forming apparatus, so that an image formed in the image forming unit is transferred to the paper sheet.
Such paper feeding devices may have installed therein a sensor for detecting the number of remaining sheets in the paper feeding device, in order to notify an out-of-paper occurrence to users, when paper runs out.
For instance, (1) in known image forming apparatuses paper is replenished into a paper feeding device by way of a manual paper feeding unit, and an accurate number of remaining sheets is grasped by counting the number of replenished paper sheets at that time.
(2) In other known paper feeding devices, the distance down to the topmost paper sheet in a paper stack is measured by a distance sensor provided above the stored paper, the thickness per paper sheet is calculated on the basis of the number of printed sheets and the displacement of the paper position, and the remaining paper amount is estimated.
(3) In another ordinary configuration of paper feeding devices, a paper stack is placed on a lifting plate, and a pickup roller feeds out, sheet by sheet, the topmost sheet on the paper stack. A motor raises one end of the lifting plate, to tilt thereby the entire lifting plate in such a manner that the topmost face of the paper stack is at all times at a position abutting the pickup roller. The inclination angle of the lifting plate varies depending on the remaining sheets in the paper stack S. The above configuration is used in proposed methods for determining the number of remaining sheets on the basis of the inclination angle of the lifting plate.